Firearms are typically designed to accept and fire ammunition of a particular size and caliber. For example, an AR-15 rifle may be designed to fire a .223 Remington caliber cartridge or a 5.56×45 mm NATO cartridge. However, when shooting a significant number of rounds, it may be desirable to use a less expensive type of ammunition, such as a .22 caliber cartridge. Typically, a .22 caliber cartridge will not fit into the magazine and firing action of the firearm designed to fire another size and caliber cartridge. In addition a .22 cartridge is a rimfire cartridge and a 5.56×45 mm NATO cartridge is a centerfire cartridge which require different firing actions. Therefore, a conversion device to allow a firearm to shoot another size, type, and caliber cartridge is desirable.
In addition, in existing systems, conversion devices include slide rail legs, or the equivalent, often flex away from a chamber insert during use which can open up the tolerances between the bolt and the slide rail and allow the bolt to move off center in the firearm. Other drawbacks of existing systems also exist.